Corrosion Failure Analysis of 316L Stainless Steel Capillary for a Producing Well in an Offshore Oil Field

The corrosion failure reasons of 316L stainless steel capillary for a producing well in an offshore oil field were investigated through macroscopic reasons, chemical composition analysis, mechanical property testing, metallographic analysis and scanning electrical microscopy. The results show that the corrosion morphology of failed sections of the straight seam welded capillary exhibited outer uniform corrosion and knife-line corrosion cracking. A section of the pipe showed outer uniform corrosion where the wall thickness was decreased by nearly 30% . After the corrosion test according to GB/T 4334-2008 with uncorroded intact samples, the appearance of the weld heat affected zone(HAZ) displayed the same surface groove profile as that of the field sample. This shows that the main reason of corrosion failure was the presence of intergranular corrosion susceptibility in the HAZ of the weld of the intact samples. The groove was easy to crack when the demulsifier was injected into the capillary in the pressurized conditions after corrosion. The EDS results obtained from the microscopic view from the HAZ demonstrates the presence of Cr segregation in the austenite grain boundary region. Some suggestions and improvement measures are put forward according to the results of failure analysis.